Prime Minister Rishi Sunak of Britain took an unaccustomed victory lap on Monday, visiting Belfast to celebrate the restoration of Northern Ireland’s power-sharing government. His aides struck a deal last week that brought the North’s disaffected unionists back into the territory’s assembly.
For Mr. Sunak, who is embattled on so many other fronts, it was a rare unalloyed success — significant not just because it ended two years of political stalemate in Northern Ireland, but also because, some analysts believe, it could shore up a United Kingdom that has seemed in danger of spinning apart since Brexit.
With the revival of self-government in Northern Ireland, diplomats and analysts said, the spotlight will shift away from the tantalizing prospect of uniting the North with the Irish Republic and shine on everyday issues like cutting waiting times at hospitals or giving pay raises to public workers.
“There was a head of steam building on the issue of Irish unity,” said Katy Hayward, professor of politics at Queen’s University in Belfast. “Nothing was working, everything was broken, so people were thinking about the alternative. If you have the institutions working, it relieves the pressure a little.”
None of this is to say that the dream of a united Ireland has slipped away. Sinn Fein, the Irish nationalist party, has the largest number of seats in the assembly, a status that allowed its leader, Michelle O’Neill, to be installed on Saturday as the first minister in the government, a moment laden with symbolism. She said she could foresee a referendum on unifying Ireland within the next decade.
For the first time since the 1921 partition that has kept the North under British rule, Catholics constitute a plurality of the population in the territory. In the South, polls suggest that Sinn Fein, which has vestigial ties to the Irish Republican Army, could vault into government after elections next year.
Still, Ms. O’Neill did not mention Irish unification in her remarks after becoming first minister. That was no accident. Her goal, analysts said, is to reassure the public that Sinn Fein — working with the Democratic Unionist Party, which favors remaining part of the United Kingdom — can govern effectively.
“It’s not in their interest to keep beating that drum,” said Bobby McDonagh, a former Irish ambassador to Britain. “The focus in the coming years will be on power sharing and making the government work.”
Mr. McDonagh said the Democratic Unionist Party, or D.U.P., had a similar incentive. Having haggled with Mr. Sunak’s government for nearly a year to improve the terms of a trade agreement Britain struck with the European Union on behalf of Northern Ireland, the party’s best argument for staying in the union is to show that it can work constructively with the nationalists.
For Mr. Sunak, a period of tranquillity would ease the anxiety that has lingered ever since Britons voted to leave the E.U. in 2016. Northern Ireland voted against Brexit by 56 percent to 44 percent, and the resulting tensions — related to its unusual trade status as part of the United Kingdom but sharing an open border with Ireland, a member of the E.U. — divided the unionists and played to the advantage of the nationalists.
That, plus the changing demographics in the North, fed hopes that Irish unity might come sooner than expected.
A similar dynamic took hold in Scotland, where fierce opposition to Brexit caused a spike in favor of breaking away from the United Kingdom (Scots voted against leaving in a referendum in 2014). But there, too, events have broken in Mr. Sunak’s favor: While support for independence remains stable at just under 50 percent, the party that drives the movement, the Scottish National Party, has hemorrhaged support since a financial scandal involving its former leaders.
In the case of Northern Ireland, diplomats say Mr. Sunak deserves credit for methodically renegotiating the arrangement left by one of his predecessors, Boris Johnson, whose exit deal with Brussels saddled the North with an awkward set of restrictions on its trade.
“What he’s been doing is undoing the damage that Boris Johnson did,” said Jonathan Powell, the former chief of staff to Prime Minister Tony Blair who helped negotiate the Good Friday Agreement, which introduced power sharing and ended decades of sectarian strife in Northern Ireland.
Mr. Powell also credited Julian Smith, a former Northern Ireland secretary, who he said carried out back-channel discussions with the unionists, as well as John Bew, a foreign policy adviser to Mr. Sunak and native of Belfast, who was deeply involved in the effort to turn around the unionists.
The British government framed its agreement with the D.U.P. as a way to ensure that Irish unification remains a distant goal. In a paper it issued on the terms of the deal, it said that, based on recent polling data, the government “sees no realistic prospect of a border poll leading to a united Ireland.”
Under the Good Friday Agreement, Britain would be obliged to call a referendum on whether Northern Ireland should leave the union if there is clear evidence that a majority support that in the North and South. In the North, polls show people would vote against leaving by a double-digit margin. In the republic, however, polls show a strong majority in favor of unification.
“We believe that, following the restoration of the devolved institutions, Northern Ireland’s future in the U.K. will be secure for decades to come and, as such, the conditions for a border poll are unlikely to be objectively met,” the government said. (Ms. O’Neill’s comment about the timing of a border poll came in response to that statement.)
Mr. Sunak, who met in Belfast with Ms. O’Neill and Emma Little-Pengelly, the D.U.P. representative serving as deputy first minister, said the deal with the unionists would secure Northern Ireland’s place in the union.
But Mr. Sunak himself faces an election later this year, which analysts said could have uncertain consequences for the stability of the new government in the North.
If Sinn Fein were to take power in the South, some analysts said, it could reinforce the resistance of some voters in the North to breaking away from the union. But it would also make the prospect of Irish unity more tangible.
“The Irish unity debate will have to become more real,” Professor Hayward said. “Everybody realizes you don’t want to repeat the Brexit experience. They will have to manage it more carefully.”